1. Field of the Invention
The present invention relates to an optical waveguide device used in optical information processing, optical communications, and so forth, and to a method of manufacturing such device. More particularly the present invention relates to an optical waveguide device that comprises silica glass and has a large refractive index difference between its core layer and cladding layer, and to a method of manufacturing such device.
2. Description of the Related Art
Flame hydrolysis deposition (FHD) and plasma CVD are two of the known methods of forming on a substrate an optical waveguide film that serves as the core layer of an optical waveguide device. FHD involves creating a porous film by subjecting silicon tetrachloride (SiCl4) to flame hydrolysis and depositing the resulting silica glass microparticles on a substrate, and then consolidating this porous film at a high temperature of at least 1000° C., thereby obtaining an optical waveguide film. Plasma CVD involves generating a plasma in a vacuum vessel, activating within this plasma a reaction gas introduced into the vacuum vessel, and depositing an optical waveguide film on a substrate that has been placed in the vacuum vessel.
There are two known types of plasma CVD methods, depending on how the plasma is generated. The first method is parallel plate plasma CVD, which is disclosed in Japanese Patent Application Laid-Open NO. H08-262260. In this method, a substrate is placed between an upper electrode plate and a lower electrode plate disposed parallel to each other in a vacuum vessel, and high-frequency power is supplied to either the upper or lower electrode plate, which generates a plasma between the upper and lower electrode plates. The second method is inductively coupled plasma CVD, which is disclosed in Japanese Patent Applications Laid-Open H08-262250 and H10-221557. In this method, high-frequency power is supplied to a coil disposed outside a vacuum vessel, and a plasma is generated inside the vacuum vessel by the inductive electrical field thus formed.
It is important to make the size of an optical waveguide device compact. To this end, loss must be reduced by setting a great refractive index difference between the core layer and the cladding layer. However, the following problems are encountered with the methods mentioned above.
In FHD, if a core waveguide film is formed on a substrate in a manner such that it contains a large amount of additive for controlling the refractive index in order to increase the refractive index difference between the core layer and the cladding layer, the difference in the coefficients of thermal expansion between the substrate and the core waveguide film causes overall warping. Warping can lead to cracking of the substrate or to a marked decrease in the working precision of an optical circuit in the patterning of the optical circuit. Consequently, there is a limit to how much the refractive index difference can be increased. Also, in the process of consolidating the core waveguide film, because the core waveguide film is porous, and because the heat treatment temperature is 1000° C. or higher, the refractive index-controlling additive added to the core waveguide film tends to volatilize. This makes it difficult to obtain a core layer with a high refractive index, and the limit to the relative refractive index difference of the core layer with respect to the cladding layer is about 1%.
In plasma CVD, an optical waveguide film is formed on a substrate at a relatively low temperature, about 400° C., and the above-mentioned problem of warping encountered with FHD is generally considered not to apply. Also, since the core waveguide film formed on the substrate is transparent and solid, there is no need for a consolidation process, nor is there any volatilization of the additive for controlling the refractive index. Nevertheless, the inventors of the present invention found that when a core waveguide film containing a large quantity of additive for increasing the refractive index is formed on a substrate by a conventional plasma CVD method, tensile stress sometimes occurs in the core waveguide film with respect to a cross section perpendicular to the substrate.
Japanese Patent Application Laid-Open NO. H5-241035 discloses an optical waveguide device in which the core layer is made of a glass other than silica glass (that is, mainly made of SiOxNyHz), and the refractive index difference between the core layer and the cladding layer is set high.